This invention relates to satellite communications systems generally, and more particularly to satellite communications systems in which uplink antenna beam nulling is used to reject interference from a particular location within the beam.
European Patent Office Publication No. 414,534 A2, based upon U.S. patent application Ser. No. 397,229 filed Aug. 23, 1989 in the name of Benden, describes an anti-intrusion defeating arrangement for communications satellites. As described therein, commercial communications satellites require inexpensive, lightweight, and low power consumption techniques for combating uplink interference. As described in the publication, each feed horn of the receive antenna receives signals from earth stations in different regions of the coverage area. Each horn can receive up to twelve channels from its own particular region. The output from each of the horns is applied by way of a hybrid power divider to a power combining system, and then to a low noise amplifier (LNA) and converter. The signal divided off at the hybrids is applied to a further variable power combining system and to a second LNA. Thus, there is a "main" and an "ancillary" system, and each channel is available from a main LNA and from an ancillary LNA. An array of single pole, double throw (SPDT) switches selects either the main or ancillary channel output for each channel, and the channels are combined for re-transmission to the earth by an output multiplexing arrangement. The assumption is made in the Benden publication that an interfering signal in a particular channel originates from a feed horn other than the feed horn which receives the desired signal which is interfered with. With this assumption, (a) the variable power combiners in the ancillary system are adjusted to combine signals only from the horn receiving the desired signal in the ancillary channel to thereby reject the interfering signal in that particular channel, and (b) an appropriate switch of the array of SPDT switches selects the desired channel from the ancillary system in which the interfering signals is rejected, not from the main system. The above-described Benden system has the disadvantage that the received signal is power divided in hybrid circuits before low noise amplification, and thus reduces the amount of signal available before low noise amplification, which undesirably degrades the gain-to-temperature ratio (G/T) in every channel. If the ancillary-system signal is 7 dB below the main system signal, the through loss in the main channel, which is the degradation of G/T, is almost 1 dB. This degradation of the G/T could be avoided by placing an LNA within each channel, but this increases the complexity and weight of the system, and decreases its overall reliability, especially in view of the fact that the low noise amplifier is ordinarily redundant. Thus, if twelve communications channels are provided, twelve LNAs would be required in each of the main and ancillary systems, together with an additional twelve redundant units in each of the main and ancillary systems.